The need for rapid heat extraction from foodstuffs, human tissues and fluids is well documented.
Direct-contact heat transfer liquids such as liquid nitrogen and liquid carbon dioxide are well known and are used in extremely low temperature applications such as cryogenic freezing. Such freezing requires expensive equipment to maintain the liquid state of the coolant by the proper combination of pressure and low temperature to prevent evaporation and consequent loss of vaporized heat transfer liquid to atmosphere. Since there is a direct relationship between frozen food that tastes fresh and the rate at which the food is frozen, some of the more expensive foods are cryogenically frozen with liquid nitrogen, etc. despite its high freezing costs. However, for the economical freezing of less sensitive foodstuffs, the extreme low temperatures of liquid nitrogen and liquid carbon dioxide and attendant expense of the specialized equipment to handle it are not deemed cost effective.
Another trend in the food industry is discernable. That is, institutional food preparation is now frequently prepared at least partially at a remote location ("hub" kitchen) and then transported to the site of ultimate consumption, for example at a public school. As a result, the "hub" kitchen must first properly prepare the food by at least partially cooking, and then quickly reduce the food temperature below 40.degree. F. to minimize the development of any bacteria, such as salmonella, which would adversely effect the ultimate consumer. For example, a pan having a 2 inch depth within which food has been cooked is subjected to a known prior art refrigeration device whose manufacturers claim a drop in temperature from 180.degree. F. to 40.degree. F. in approximately 90 minutes. While in some instances this may be possible, certain dense foods will still have a core temperature in excess of 40.degree. well after the manufacturer's suggested allotted time in these commercial freezers, providing an unacceptable bacterial risk.
Some institutional kitchens will accelerate the chilling process by removing the lid which covers the cooked food. This allows heat and vapor to leave the tray in order to accelerate the chilling process. Clearly, vapor loss ultimately will dehydrate the food, at least along an outer periphery, and subsequent freezing provides the phenomenon known as "freezer burn". More importantly, the open cooking trays serve as an incubator for airborne bacteria which increases the likelihood of communal contamination which exacerbates the problem.
Similarly, blood plasma contains proteins which are labile and decay at all temperatures above -30.degree. C. This decay is occurring at the highest rate in the temperatures above fusion (about -2.degree. C.). Consequently, the faster the plasma can be frozen, the greater the quantity of these proteins that will remain in the plasma for therapeutic purposes. The importance of rapid freezing of blood plasma can be best illustrated by noting that the most labile protein, the clotting Factor VIII, is often reduced in content by 50% during the elapsed time between collection from the donor and being frozen to -30.degree. C. Nevertheless, the Factor VIII concentrate which finally survives the manufacturing process has a retail market of 600 million USD annually.
Most conventional prior art food and blood products freezers comprise refrigeration units in which the heat transfer medium is air- a gas. Although it is well known that liquids are more efficient than gases as heat transfer fluids, when freezing or chilling foodstuffs or blood products, direct contact with the heat transfer fluid is acceptable only if the fluid is substantially non-toxic and/or has tolerable levels of migration of toxins to food, and/or does not lend an acceptable taste or texture to the foodstuffs.
Because of their intrinsically low toxicity, as well as their low boiling points, minimal density and high volatility, the normal refrigerating gases such as LN.sub.2, CO.sub.2 or mechanically refrigerated air will pass no toxins on to the items to be chilled or frozen.
Pure water, the most obvious, inexpensive and excellent heat transfer fluid is unusable as a means of freezing foodstuffs or blood products because water freezes at 0.degree. C. (32.degree. F.) and most blood products and foodstuffs freeze between -1.degree. C. and -4.degree. C. (31.degree. F. and 24.degree. F.). Since the rate of heat transfer is proportional to the temperature difference between the heat transfer liquid (which is cold) and the warmer foodstuffs or blood products, only liquids which remain low in viscosity (i.e. pumpable) down to at least -20.degree. C. or -30.degree. C. (4.degree. F. to -22.degree. F.) can rapidly freeze foodstuffs or blood products which freeze at approximately -2.degree. C. to -4.degree. C. (30.degree.-24.degree. F.).
Recognizing the intrinsic heat absorbing superiority of liquids over gases, refrigeration specialists have attempted to reduce the freezing point of water by adding salts or glycols to water. For example a 50/50 mix of glycol and water has a freezing point of -30.degree. C. (-22.degree. F.) and a 25/75 mix of calcium chloride and water has a freezing point of -29.degree. C. (-21.degree. F.) thus allowing them the ability to rapidly freeze foodstuffs or blood products. However, the addition of these freezing point lowering chemicals to water has some serious drawbacks.
Both calcium chloride and propylene glycol have levels of toxicity that are cause for concern. In fact in the food industry, any foodstuffs coming into contact with water containing any percentage mix of glycol or chloride must be protected by a water proof, FDA approved plastic cover. In the blood products industry, where health concerns are even more acute, no such mixtures are currently in use.
A further problem with such heat transfer mixtures is that they become viscous at low temperatures and thus cling to the surface of a packaged foodstuff and are carried out of the freezing bath on the surface of the packaging. This surface contamination is both messy and toxic and consequently must be washed off with pure water before continuing on for further handling. Since by definition this washing must be done by water warmer than 0.degree. C., this cleansing process adds heat to the frozen product, which is not desired, and creates a new waste fluid which must be disposed of in an environmentally suitable manner.
A further problem is that when food items are introduced to a chamber where the chloride or glycol mixture resides, it is virtually impossible to stop certain amounts of moist, outside air from invading the chamber. Once inside, the moisture in the air condenses out and goes into solution with the chloride or glycol mixture, thereby continually altering the percent water in the mixture. Since the freezing point rises with the increasing percentage of water, continual monitoring and adjustment of the mixture must be maintained.
Chlorofluorocarbon refrigerants such as the Freon (trademark of the DuPont Company) compositions have previously been employed in closed loop non-direct contact refrigeration systems in which the circulating refrigerant is never permitted to come into direct contact with the articles to be chilled. Toxins present in refrigerants of this type have, with two exceptions noted below, prevented these refrigerants and/or solvents from being approved by regulatory authorities such as the United States Food and Drug Administration (FDA) for direct contact with foodstuffs.
To date it is believed that only two chlorofluorocarbons, CFC 12, (dichlorodifluoromethane) and the proprietary mixture of applicant's called Instacoolant.TM. containing CFC-113, have ever been approved by the FDA for direct contact with human food. The CFC-12 composition is, however, only marginally suitable for use in immersion or spray contact freezing of foodstuffs because of its relatively low boiling point (-30.degree. C.) which results in the high loss of CFC-12 to the atmosphere despite expensive recovery systems and the consequent expense of regular replacement of lost fluid. A suitable direct contact heat transfer liquid must therefore also have a suitably high boiling point above normal ambient temperatures, preferably above 50.degree. C., in order to maintain tolerable losses due to evaporation.
The following patents reflect the state of the art of which applicant is aware, in so far as these patents appear germane to the instant process, particularly as dictated by the claims. It is respectfully submitted that these patents neither teach singly nor render obvious when considered in any conceivable combination the claimed nexus of applicant's invention. Moreover, these patents are included to discharge applicant's acknowledged duty to divulge known prior art.
______________________________________ INVENTOR PATENT NO. ISSUE DATE ______________________________________ H. G. Vore 2,274,284 Feb. 24, 1942 H. Y Stebbins 2,286,514 June 16, 1942 F. H Clarke 2,914,445 Nov. 24, 1959 C. A Mills 3,027,734 April 3,1962 W. L. Morrison 3,090,134 May 21, 1963 W. L. Morrison 3,096,627 July 9, 1963 S. S. Thompson 3,440,831 April 29, 1969 Schwartz 3,753,357 Aug. 21, 1973 Howard 3,774,524 Nov. 27, 1973 Faust, et al. 3,875,754 April 8, 1975 Pert, et al. 4,251,995 Feb. 24, 1981 Douglas-Hamilton 4,530,816 July 23, 1985 Bilstad, et al. 4,630,448 Dec. 23, 1986 Sakai 4,689,963 Sept. 1, 1987 ______________________________________
Each of these patents can generally be characterized with respect to one type of infirmity or another discussed generally, supra. For example, many of the systems included to show the state of the art are brine or glycol type systems. Thus they may cause solutions to adhere to the chilled article. One brine system, Stebbins uses a pliant conveyor with an optional vacuum. Sealing problems may attend this structure. Another infirmity with respect to some of the prior art is that they have the expense associated with liquid nitrogen or carbon dioxide systems. All systems that utilize cloroflurocarbons (CFC) suffer from the malady that the CFCs are susceptible to vaporization and escape into the atmosphere. Another infirmity is that some of the prior art involving plastic covering and medical products do not contemplate any form of heat extraction.